Device for the selective detection of objects

ABSTRACT

A device for the selective detection of objects, such as aircraft, missiles, helicopters and the like, by detecting outgoing rays emitted from the objects, preferably infrared rays, comprises at least one sensor unit with two optical entrances arranged at a distance relative to each other across a sight line from the device to the object. The sensor unit includes at least one focusing arrangement for focusing the rays onto a focal plane. The sensor unit includes at least one radiation sensitive detector element arranged to emit signals corresponding to the amount of radiation. The device is arranged to variably scan an angular field in azimuth and/or elevation. The device further includes an evaluation unit arranged to receive the signals. The evaluation unit is arranged to select objects which seem to be of similar size with respect to the angle measured from the position of the device, on the one hand large objects being at a long distance, such as aircraft, and on the other hand small objects being at a short distance, such as birds, by suppressing signals whose amplitude is a function of the scanned azimuth or elevation angle showing a relatively large size compared with signals showing a relatively small size.

TECHNICAL FIELD

The present invention relates to a device for the selective detection ofobjects, such as aircraft, missiles, helicopters and the like bydetecting rays emitted from the objects, preferably infrared rays.

BACKGROUND OF THE INVENTION

The invention is based on a known technique, which will here bedescribed briefly herein below.

On the market there is a device, a so-called scanner, intended fordetecting flying objects by means of IR-rays emitted from the objects.Such a scanner comprises a sensor unit, an evaluation unit and a displayand control unit. The sensor unit receives IR-rays from the objects tobe detected within the momentarily scanned solid angle. The sensor unittransmits the corresponding signals to the evaluation unit, whichprocesses the signals and upon the significant detection of objects,i.e., in a military context, targets, transmits the correspondingsignals including direction coordinates to the display and control unit.From it the collected output data go, which in a military context partlytakes the form of a visible picture on a screen, and partly the form ofsignals, may be forwarded online and e.g., be used for guidIng afire-control system for anti-aircraft defense.

The evaluation unit has as its purpose to select those signals, amongthe signals received from the sensor unit, which are significant, i.e.,which indicate targets within the scanning range of the sensor unit, andto indicate when such targets appear, and generally to indicate theircoordinates. The evaluation unit functions thus according to preselectedprogrammed criteria of what will be considered significant objects,i.e., targets.

The evaluation unit has a filter function and a decision function. Thesignals which are received from the sensor unit (measured intensity as afunction of direction) are inputted to the filter which is designed toenhance the typical signals of targets. During filtering the directionalinformation is retained. A resulting filter output signal for a certaindirection is a measure of the probability that there is a target in theactual direction.

A concrete example is a filter, which for every direction forms thedifference between measured intensity in the actual direction and theaverage intensity in a two-dimensional interval of the surroundingdirections. Typical for a filter in this application is particularlythat the output signal for a certain direction is a weighted sum of theinput signals of the filter in an angular range in and about thatdirection.

In a scanner whose purpose is to select significant objects, i.e.,targets, in addition to the filter, a decision function is alsorequired. That function decides whether a significant object exists ornot. The most common decision function is to compare the output signalof the filter with a threshold level. If the threshold is exceeded, asignificant object, i.e., a target, is indicated. The mechanism whichchooses the threshold level may also be included in the decisionfunction. The threshold level is often determined through statisticalevaluation of the output signals of the filter within a large range,possibly the whole scanning range. The object is to find a level whichis exceeded at an acceptably low frequency in the absence of significantobjects, i.e., targets (false-alarm frequency) and which yet is not toohigh for appearing targets to be indicated with certainty.

Scanners of the type described aboved function preferably within theIR-spectral ranges 3 to 5 and 7 to 13 micrometers, respectively, whichrepresent "windows" with regard to the transmission spectra of theatmosphere for IR-radiation. That means that the focusing means of thesensor unit, which in itself can consist of a lens or mirror, usuallyconsists of a silicon lens for the 3 to 5 micrometer range and agermanium lens for the 7 to 13 micrometer range, i.e., it is chosen withrespect to the actual spectral range. In view of hitherto existingcorresponding radiation-sensitive detector elements, such a lens must bemade comparatively large, in order that the sensor unit will producesignals such that the evaluation unit can detect significant targetswith any appreciable precision.

Such a scanner cannot distinguish between birds (i.e., insignificantobjects) at a relatively close distance and aircraft (i.e., significantobjects) at a longer distance) which means that birds could cause afalse alarm which is a great disadvantage in hitherto known scanners ofthe type described above.

It is therefore an object of the present invention is to design a devicewherein it is possible to distinguish between relatively close objectsfrom which radiation is emitted, and such objects which are located at alonger distance, but which objects when observed subtend a similar solidangle.

SUMMARY OF THE INVENTION

The device according to the present invention comprises at least onesensor unit, whereby the device is arranged to receive the outgoing oremitted rays through at least two optical entrances arranged at arelative distance across a slight line from the device to the object.The sensor unit includes at least one focusing means arranged to focusthe rays onto at least one corresponding focal plane. The sensor unit isequipped with at least one radiation-sensitive detector element beingpositioned in a focal plane; the detector element is arranged to emitsignals corresponding to the incoming radiation. The device is arrangedto variably scan an angular field in azimuth and/or elevation. Thedevice further comprises an evaluation unit, arranged to receive saidsignals.

According to the invention, the evaluation unit is arranged to selectobjects which seem to be of similar size with respect to the anglemeasured from the position of the device, on the one hand large objectsbeing at a long distance, such as aircraft etc., and on the other handsmall objects being at a short distance, such as birds etc., bysuppressing signals whose amplitude as a function of the scanned azimuthor elevation angle shows a relatively large size compared with signalswhich show a relatively small size.

In one embodiment of the device according to the invention the devicecomprises at least two sensor units, having their optical entrances forthe outgoing rays from the objects at a relative distance across thesight line. In this case, each sensor unit functions in itself as acomplete unit. Naturally, a corresponding signal processing in theevaluation unit is required, i.e., the signals from the sensor unitsshall be added before effecting further signal processing in theevaluation unit. It is suitable to design a sensor unit so that eachoptical entrance comprises a deflection means, preferably a mirror,arranged to deflect the incoming rays at the entrance to thecorresponding focusing means. If one mirror per optical entrance is usedas a deflection means, the mirrors used must naturally be so arrangedthat the radiation from each mirror, usually positioned at an angle of45° to the incoming rays, can in fact reach the focusing means. This isachieved through an arrangement wherein the apertures of the opticalentrances are so displaced not only in a first direction relative to thesight line, which is a condition for the proper function of the device,but also in a second direction perpendicular to the first direction. Itis also conceive to use mirrors which are partly transparent to theactual radiation. If it is desired to have the optical entrancesarranged in two groups, one on each side of this focusing means, anextra mirror per group may be arranged to guide the radiation, which istransmitted from the group, to the focusing means. That is usually alens, as in the case of IR-radiation made of e.g., germaniums, butconcave mirrors are also possible, e.g., according to theCassegrain-system. Other defection means can be used, such as prismsarranged to totally reflect the incoming rays. The optical axes of theoptical entrances shall be parallel.

The device according to the invention may under certain circumstances,include one single detector element but it is preferred to use aso-called array consisting of a number of detector elements arranged ina row. Such detector elements can also be arranged in a plane, i.e.,two-dimensionally.

The device can be designed so as to be able to scan a small or largesolid angle in azimuth and elevation. The detector elements of thesensor units, either one single or several elements arranged either as aone-dimensional or two-dimensional array, call for differentarrangements for widening the viewed angle in azimuth or elevation.

In a suitable embodiment of the device according to the invention thedevice as a whole is rotatable about an essentially vertical axis,whereby a number of optical entrances are arranged at a distance fromsaid axis, across the sight line toward an imaginary object. The deviceis further movable in elevation e.g., step-by-step, so that it e.g.,rotates one about in every chosen elevation position. The movement inelevation can alternatively be continuous.

The scanning range can be arbitrarily large or small provided that thescanning can be effected across the object in at least one directionwhich lies within the plane where the object and al least two opticalentrances are situated.

An angle position transducer for azimuth and elevation provides aposition signal to the evaluation unit for every instantaneous measuringdirection.

Instead of rotating the device it can be made to move e.g., forward andbackward.

It is further possible to let the device as a whole be immobile andinstead let the sensor unit move or, where appropriate, the sensor unitsinclude an optical scanning means, arranged to variably scan an opticangle, in addition to the angle which is scannable with thecorresponding detector element, in azimuth and/or elevation. One way ofachieving this is to arrange the deflection means, e.g., the mirrors, tobe movable. In an extreme case a sensor unit can be arranged with onesingle detector element, having a scanning means, such as a mirrorfunctioning as a deflection means, which mirror is movable about twoaxes perpendicular to each other. Such an arrangement, however, shouldhave a limited practical applicability, even with a correspondingevaluation unit. In such a device better performance can be achieved ifthe detector elements are arranged as an area array, i.e.,two-dimensionally.

In a preferred embodiment of the invention there are provided opticalentrances arranged in groups. These groups can suitably comprise twooptical entrances each. The distance between the optical entrances in agroup will thus be shorter than the distance between two opticalentrances belonging to different groups. Through such an arrangement,further suppressing of signals which originate from comparatively small,comparatively close objects is achieved.

The invention will now be described more in detail with reference to theaccompanying figures which relates to an example of one embodiment ofthe device, i.e., a scanner designed for military use with the aid ofIR-technique.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of the scanner,

FIG. 2 is a perspective view of the sensor unit in the scanner,

FIG. 3 depicts the optical arrangement of the sensor unit,

FIG. 4 depicts focusing means and detector elements,

FIG. 5 is a linear array of detector elements,

FIG. 6 diagrammatically depicts the scanning method of the device,

FIG. 7 is a block diagram of the sensor unit,

FIG. 8 is a block diagram of the evaluation unit,

FIG. 9 depicts a sensor head, diagrammatically seen from above,

FIG. 10 is a graph showing signal amplitude via the four opticalentrances in FIG. 9 as a function of the azimuth angle,

FIG. 11 is a graph showing detector output signal with four and oneoptical entrances, respectively,

FIGS. 12, 13, and 14a and 14b are graph showing a detector and filteroutput signal with four and one optical entrances, respectively, atdistances of 500 m, 130 m and 10 km, respectively,

FIGS. 15 and 16a and 16b show diagrammatically the function of a filter.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, there is shown a sensor unit 1, an evaluation unit 2 and adisplay and control unit 2. IR-rays received from objects within thescanning range generate corresponding signals 5 transmitted from thesensor unit to the evaluation unit wherein evaluated signals 6 forsignificant objects are generated and transmitted to the display andcontrol unit 3. Collected output data 7 from unit 3 may take the form ofa visible picture on a screen and also signals for e.g., guiding afire-control system for anti-aircraft defense. The connection designatedby 8 transmits starting and stopping commands etc., to the units 1 and2.

The sensor unit 1 (FIG. 2) comprises a sensor head with apertures 10defining four optical entrances. The sensor head is suspended from anelevation servo 11, for a step-by-step adjustment into differentelevation angles by turning about a horizontal axis. The elevation servo11 is rigidly connected with a vertical axis 12, supported on a stand13. A motor within the standis arranged to drive the axis 12 with aconstant number of revolutions per second. This in addition there is anangle transducer and slip rings required for signal transmittancethrough a cable 14 to the other units.

The optical arrangements in the sensor head 9 can be seen in in FIG. 3wherein a focusing means includes a lens 15. In its focal plane a numberof detector elements are arranged into an array 16 with its axis 17perpendicular to the horizontal principal axis 18 of the lens. On theother side of the lens 15 four plain rectangular mirrors 19 arearranged, whose symmetry axes 20, which are parallel to the short sidesof the mirrors, intersect the principal axis 18 of the lens and areparallel to the axis 17 of the detector array 16. The mirrors areparallel relative to each other and the mirror plane forms an angle of45° to the plane defined by the axes 17, 18. The mirrors 19 are sodisplaced relative to each other along the symmetry axis 20, and are ofsuch a size that partly the effective total aperture of the sensor unitis principally the same as the area of the lens, and partly, all themirrors contribute principally equally to said apertures. The opticalaxis 21 of the sensor head is defined as the axis which interacts theaxes 18, 20 at right angles by the mirror which is the closest to thelens. It is understood that an object on the optical axis 21 at a tongdistance will be reproduced as a dot at the opener of the detector array16.

In FIG. 4 the focusing means, i.e., the lens 15 and the detector array16, is drawn in with realistic size relationship. The detector array hasa length 1 and the lens a focal length f. In the figure is drawn raystraced from three different small objects being at a long distance andso that they are reproduced in the extreme positions and the center,respectively, of the detector array.

It is understood that the field of view α_(e) those of the sensor unit 1in the shown plane is

    α.sub.e =

In this example α_(e) =0.16 radian or 160 milliradians.

In FIG. 5 the appearance of the detector array is shown in more indetail. In this example there are 64 similar separate detector elementsof a size a×b. In this example a=0.2 mm and b=0.5 mm. The length of thedetector array is thus 64×0.5 mm =32 mm. As a consequence the focallength of the lens is 200 mm. Each detector element covers a solid angleof 2.5 mrad =1 mrad.

With respect to FIGS. 2 to 5 it is evident that the sensor unit canmeasure simultaneously the incident-ray intensity in 64 directions 2.5mrad from each other within a sector of 160 mrad in the vertical plane,whereby the resolution is 2.5 mrad in elevation. By rotating the sensorhead about the vertical axis 12 in FIG. 2, measuring can be effected forazimuth angles throughout the whole revolution with an angularresolution of 1.0 mrad. Between different revolutions the elevation ofthe sensor head is altered with the aid of the elevation servo 11. Acomplete scanning can e.g., comprise three revolutions, i.e., a scanningof a range of elevation of 540 mrad or about 31°. Such a scanning cycleis shown in FIG. 6.

A block diagram of the sensor unit is shown in FIG. 7. Here, the sensorhead is designated by 9, whereas an azimuth motor is designated by 22,an elevation servo by 11, an azimuth angle transducer by 23, the mirrorsby 19, the lens by 15, the detector array by an amplifier by 24, amultiplexer by 25 and an A/D-converter by 26. The sensor unit emitsoutput signals 27, 28.

The signal 27, then, is a digital signal in series form which in acertain sequence and with a certain scale factor designates theincident-ray intensity measured via the respective detector elements.The signal 28 shows the azimuth direction of the optical axis of thesensor head. The elevation servo 11 is controlled by a signal 29. Duringa complete scanning cycle of three revolutions the signals 27, 28 and 29describe the measured radiation intensity as a function of direction inthe whole scanned solid angle range, which, as discussed above, in thiscase covers 540 mrad in elevation and the whole revolution in azimuth.

The measuring values produced by the sensor unit inputted to theevaluation unit during a scanning cycle, can mathematically be said todescribe a matrix, here designated by A, whose elements a_(ij) designatethe measured incident radiation intensity in the direction

    azimuth=i×0.5 milliradians

    elevation=j×2.5 milliradians

relative to a chosen reference direction. The matrix will be referred tobelow in connection with an example of filter operation.

The signal from each separate detector element is read with a spacing inazimuth which is similar to half of the angle width of the detectorelement, i.e., 0.5 mrad. Totally, a large number of measured values willthus be emitted from detector elements in the form of digital signals tothe evaluation unit, where they are stored in a memory, and can bevisualized on e.g., a cathode-ray lube in such a way that the pictureshows a plane picture of the scene which is covered by the scannedrange. In the picture the luminous intensity in a certain point is ameasure of the measured incident IR-radiation intensity in the measuringdirection corresponding to the direction of the point.

The evaluation unit 2 of FIG. 8 is in the form of a block diagram andincludes a memory 30 (which in this connection is called image memory),a filter 31, a threshold calculator 32, a comparing means 33 and atarget memory 34. Via the sensor unit the digital signals from thedetector elements are available. In the image memory 30 differentcombinations of digital signals from the detector elements are storedtemporarily, according to a certain sequence, thus representingdifferent parts of the scanned angular range in such a way that during ascanning cycle, the signals from all the parts of the scanning range canbe processed by the filter 31, which calculates, in a known manner, thedifference between the signal intensity in a chosen direction and thesignal intensity in the area surrounding it, area by area of the scannedrange.

To determine whether a significant object has been measured or not, adecision function is now used, comprising the threshold calculator 32and the comparing means 33, which comparing means also has a directconnection with the filter 31.

The evaluation function will now be shown by example.

In FIG. 9 there is depicted (vertically from above) the mirrors 1, thelens 15, the detector element 16, an object 36, the optical axes of theoptical entrances 37-40 and rays from the objects to the opticalentrances 41-44. This in the figure a scanner is shown in the scanningposition β=O, i.e., azimuth position O. Scanning is effected by means ofrotation in the clockwise direction (i.e., arrow 45). The diameter ofthe lens is D_(opt), and the distance between the mirrors is d₁₂, d₂₃,and d₃₄. The angular distance of the object from the respective opticalaxis is β₁, β₂, β₃ and β₄ and the distance to the object is R. The sizeof the object is D_(obj).

As an example of measured lest results, some diagrams are given belowwhich show the function of the device

The device has in this example the following dimensions:

    ______________________________________           D.sub.opt   = 0.20 m           d.sub.12    = 0.25 m           d.sub.23    = 0.50 m           d.sub.34    = 0.25 m           β.sub.1                       = 2.00 mrad    ______________________________________

if R is varied from 90 m to 10 km a number of diagrams will be obtained,where the vertical axis in all the oases is related to the signalamplitude.

As is evident from the choice of d₁₂, d₂₃ and d₃₄, the mirrors in thisexample are arranged in two pairs with regard to the distance Thedistance within each pair is 0.25 m. The distance between the pairs is0.50 m, measured as the distance between the middle two of the four. Theobjective is to prove the effect of the mirrors being positioned in thisparticular manner.

When choosing the parameter values, as well as the examples ofembodiments of the scanner otherwise, the objective is not to describean optimal solution but merely to give one solution to illustrate theinvention.

The following examples, FIGS. 10-14, relate to objects which, seen fromthe device i.e., the optical entrances of the sensor unit, show asimilar solid angle extension and similar radiation intensity, i.e.,birds at a comparatively shorter distance and aircraft at a longerdistance.

In FIG. 10 there is shown, for example, at a 90-m distance, the fouroptical signals 1-4 which reach the detector via the four mirrors. Thenumbering of the signals in FIG. 10 corresponds to the numbering of themirrors in FIG. 9. It is to be observed that the output signal of thedetector is equal to the sum of those four optical signals. The outputsignal of the detector is shown in FIG. 11 (curve a). In FIG. 11 (curveb) the detector signal is shown in the case of one single opticalentrance. FIG. 11 relates to the distance 90 m, as does FIG. 10.

The functioning of the filter 31 will be described with reference toFIG. 15 the figure shows a section of the matrix A where each element inthe matrix is represented by a square check. In the check pattern twoareas have been indicated, designated core and frame, respectively. Thecore comprises three matrix elements, the element a_(ij) being in themiddle. The frame comprises the matrix elements which adjoin the coreand surround it. The filter function is to calculate the differencebetween the highest element value in the core and the highest elementvalue in the frame. The calculation is done for every possible position(i,j) in the matrix A. The result is a new matrix B with the elementsb_(ij), which is the output signal of the filter.

The function of the filter 31 is illustrated in FIG. 16a and and 16b.Here the numerical values of the elements in the matrix A have beenwritten. In FIG. 16 a the highest value in the core is and the highestvalue in the frame is 3, and so the output signal of the filter is zero.This in FIG. 16b the highest value in the core is 3 and in the frame 1and the output signal is then 2. That shows that the filter tends toenhance, i.e., provide a larger output signal for objects which in thematrix A have a size which is smaller than or similar to the size of thefilter core, whereas larger objects are suppressed i.e., give a smalloutput signal from the filter.

It is to be noted, that it would be sufficient to use a filter whichweighs together only the signals from the plane which contains theobject and the optical entrances. In the example shown the filter wouldbe illustrated by row No. i.

In FIG. 15 the size of the filter core has been given, in azimuth 1.5mrad and in elevation 2.5 mrad, which values follow from earlier givensampling intervals 0.5 mrad and 2.5 mrad in the respective directions.It can he pointed out here, that an aircraft at a distance of 10 km,seen from the scanner, subtends an angle of generally less than 1.0mrad, and therefore the aircraft is well within the filter core and willtherefore be enhanced by the filter.

It is evident from FIG. 11 that for the distance 90 m, the amplitude ofthe detector signal will be considerably smaller with four entrancesthan with one single entrance. The radiation energy, received by thedetector is the same in both cases, but in the case of four entrancesthe energy is received within a larger angular range, i.e., at a certainscanning scale, during a prolonged time and therefore at a lower levelof power. The output signal of the detector or, more equally, the outputvoltage in volts, is always proportional to the incident power in wattsthe received energy is represented in figure 11 by the area below therespective curves.

It is to be noted that FIG. 11 illustrates a situation where the use offour parallel and laterally separated optical entrances results in thesuppression of the signal from an object at such a distance from thescanner that the rays from the object reach the entrances of the scannerdivergently. By divergent rays it is thereby understood rays with arelative difference of angel which is the same or larger than not toosmall a fraction of the angular resolution, which in this case is 1mrad.

In FIG. 12, for a distance of 500 m, there are shown partly the detectorsignal 1, partly the filter output signal 2 curves a1 and a2respectively depict the detector signal and filter output signal in thecase of four entrances and curves b1 and b2 depict the detector signaland filter output signal with respect to one signal entrance. FIG. 12illustrates the function of the combination of the four opticalentrances on the one hand, and of the filter on the other hand, from thefigure it is evident that the filter reduces the signal levelconsiderably more in the case of four entrances than in the case of onesingle entrance. The reason is that the four laterally separatedentrances give a widening of the signal pulse and that the filter is soarranged as to give a tower output signal when the filter input signalis wider.

In FIG. 13, for a distance of 130 m, there are shown the signalscorresponding to those in FIG. 12. Here, two signal pulses are obtained,in the case of four optical entrances, one for each pair of mirrors.FIG. 13 illustrates the function of arranging the mirrors, with respectto distance, in two pairs, in the manner described above. By arrangingthe mirrors in this manner at this distance, two wide pulses areobtained which the filter tends to suppress.

In FIGS. 14a and 14b for a distance of 10 km, there are shown thecorresponding signals, as in FIG. 12.

In FIG. 14a, there is depicted curves a1 and a2 respectivelycorresponding to the detector signal and filter output signal in thecase of 4 entrances. In FIG. 4b, curves b1 and b2 repspectively depictthe detector signal and filter output signal in the case of 1 entrance.

In FIG. 14a, 14b it is evident that the output signal from the filter isalmost as large with four entrances as with a single one. The reason isthat the distance is so great relative to the length within which themirrors are positioned, that the rays from the objects reach the scannerparallel, principally. Targets can thus be detected at long distances aseffectively with four laterally separated entrances as with one singleentrance.

It should be emphasized that the described embodiment of a deviceaccording to the invention is an example. As is evident from theaccompanying claims and the description above, there are numerousvariants of devices according to the invention. for instance the opticalentrances of the sensor unit or units can be positioned vertically in arow.

Even if a digital technique is advantageous, it is also conceivable thatthe signals need not be sampled, nor need they have a digital form. Thefilter can be designed in different ways. It can e.g., be an analogfilter and function by time instead of by angle.

The transfer function of the filter can be other than that described inthe example of the above embodiment, so long as it is arranged torelatively enhance signals from the objects or parts of objects which,when observed from the position of the sensor and in the spectral rangeof the sensor, show in angular size which is less than a chosen value,but to suppress or, to a smaller degree, enhance signals from objectswhich show a greater size than that chosen.

The transfer function of the filter need not be fixed, but can bevariable for adjustment, e.g., according to the actual conditions inrelation to the objects.

The spectral range can be other than the IR-range, e.g., the UV-rangethe visible range or the mm-wave range.

What is claimed is:
 1. A device for the selective detection of objectssuch as aircraft, missiles, helicopters and the like, by means ofdetecting outgoing rays from the objects, comprising at least one sensorunit (1) including at least one focusing means (15) arranged to focussaid rays on at least one corresponding focal plane, said sensor unitincluding at least one radiation-sensitive detector element (16) whichis positioned in said focal plane, said detector element arranged toemit signals corresponding to a level of radiation of the incoming rays,said device arranged to variably scan an angular field in azimuth and/orelevation, said device further comprising an evaluation unit means (2),arranged to receive said signals, for selecting objects which seem to beof similar size with respect to the angle measured from the position ofthe device, such objects being large objects at a long distance such asaircraft, and small objects at a short distance, such as birds, bysuppressing signals whose amplitude as a function of the scanned azimuthor elevation angle shows a relatively large size compared with signalsthat show at relatively small size.
 2. A device according to claim 1,wherein said sensor unit includes a sensor head having t least twooptical entrances arranged at a relative distance to each other across asight line from the device to the object to receive said rays in thesensor unit for transfer to the focusing means.
 3. A device according toclaim 1 further including at least two said sensor units (1)respectively having optical entrances (10) positioned to receiveoutgoing rays from the objects at a relative distance across a sightline from the device to the object.
 4. A device according to claim 2,wherein each optical entrance (10) comprises one deflection means,including a mirror (19), arranged for deflecting the incoming rays fromthe entrance to the corresponding focusing means (15).
 5. A deviceaccording to claim 1, wherein said sensor unit (1) includes a pluralityof detector elements (16) arranged in one direction.
 6. A deviceaccording to claim 1, wherein said sensor unit includes a plurality ofdetector elements (16) arranged in two directions.
 7. A device accordingto claim 2, further including means for moving the sensor unit to enablethe optical entrances and their respective fields of view to movablyscan in azimuth.
 8. A device according to claim 2, further includingmeans for moving the sensor unit to enable the optical entrances andtheir respective fields of view to movably scan in elevation.
 9. Adevice according to any one of claims 1 to 6, wherein the sensor unit(1) comprises at least one optical scanning means, arranged to variablyscan an angular range including the field of view which range is therebyscannable, in azimuth and/or elevation, by the corresponding detectorelements.
 10. A device according to claim 9, further including means formoving said scanning means including said field of view, in azimuthand/or elevation.
 11. A device according to claim 1, wherein said atleast one sensor unit (1) includes one single focusing means.
 12. Adevice according to claim 2, wherein the device comprises plural opticalentrances, arranged in groups.
 13. A device according to claim 12,wherein the groups comprise two optical entrances each.
 14. The deviceof claim 1, wherein said evaluation unit means includes an image memory,a filter, a threshold calculator, a comparing means and a target memory,wherein signals from said detector element are temporarily stored in theimage memory in a sequency spatially representing different parts of thescanned angular field, said image memory thereby being in the form of amatrix and wherein said threshold calculator and comparing means areconnected to each other and the filter to calculate the differencebetween the signal intensity in a predetermined direction and the signalintensity in an area surrounding it, area by area of the scanned range,with the filter calculating the difference between values representativeof signal intensities stored in the matrix, the calculate differencesbeing output signals stored in the target memory in cells of the matrixcorresponding to the spatially relevant cells of the image memory.